The instant invention relates generally to centrifugal pumps and more particularly to pump diffuser means having recirculation around the diffuser vanes.
One of the Navy's major concerns in the design of its numerous types of ships and submarines is the reduction of radiated noise during operation to avoid detection by the enemy. It is known that centrifugal pump tonal noise contributes significantly to the radiated noise signature of the Navy's ships and submarines.
While the design of quiet impellers for single-stage volute pumps is well established and used, a significantly more complex area of pump quieting technology is involved in multistage diffuser-type pump design. A multistage diffuser-type pump is characterized by its producing several impeller and diffuser vane-related tones rather than the single, vane frequency typical of the single-volute pumps.
Conventional multistage diffuser-type pumps have a vaned impeller and a plurality of diffuser vanes at the periphery of the impeller for directing the flow of fluid to the next stage. The diffuser vanes fit tightly between the parallel shrouds of the pump casing. This design results in a pump head-capacity curve which may droop close to and at flow shutoff. Another problem with this design is that pure-tone sounds are generated at the impeller vane passage frequency and at other frequencies not multiples of pump rotation.
It is desirable to design a centrifugal pump having a continuously rising head-capacity curve, (i.e. without droop near flow shut off) so as to be capable of stable parallel operation with another pump, and to have minimum vane-related tonal sound levels.